The invention relates to a liquid crystal display (LCD) device and a driving method thereof, and, in particular, to a LCD device and an image display method applying black/gray insertion technology.
In order to enhance the display quality of a liquid crystal display (LCD) device, a display duty cycle may be shortened by adding a sub-frame. This method is sometimes referred to as impulse-like LCD technology. Specifically, a normally black sub-frame is often added and the technology is referred to as black insertion or gray insertion technology.
In the conventional image display method of a LCD device, pixels on a display surface in the same frame are commonly driven by either a dynamic mode (e.g., pixel data changes in sequential frames) or static mode (e.g., pixel data does not change in sequential frames) according to a dynamic or static property of the frame. The black/gray frame inserting method is used when the pixels are to be driven by the dynamic mode. Black/gray frame insertion helps avoid blurring that may be associated with dynamic modes and LCDs, as is commonly understood by those of ordinary skill in the art. Such blurring is not typical with impulse devices such as cathode ray tubes (CRTs). No dark frame is inserted when pixels are driven by the static mode, in contrast to dynamic mode, because blurring is typically not an issue with motionless situations (e.g., when there is little difference between successive frames).
As shown in FIG. 1, two adjacent pixels 101 and 102 respectively receive gray-scale data A and B and display the gray-scale data A and B in the same frame time Tf. A first conventional image display technology of the impulse-like LCD device, designed to address the aforementioned blurring issue, is shown in FIG. 2. As briefly described above, a normally black sub-frame with gray-scale value of 0 is added when the pixels 101 and 102 receive gray-scale data A and B respectively, wherein traditional image frequency doubling technology is applied. Thus, pixels 101 and 102 only display the sub-frames with the gray-scale data of A and B in the first half-frame time and display the black frames (corresponding to the gray-scale value 0) in the second half-frame time, as shown in FIG. 2. The black frame inserting method can effectively halve the blur width, as judged according to traditional eye-tracking models that are associated with LCDs operating in dynamic mode. However, due to the black frame insertion, the frame luminance is halved and the image quality is lessened.
To avoid this reduction of frame luminance, a second conventional image display technology for impulse-like LCD devices has been implemented. As shown in FIG. 3, when pixels 101 and 102 receive gray-scale data A and B, respectively, the second conventional method enables the pixel 101 to display the sub-frames A′ and C in sequence and the pixel 102 to display the sub-frames B′ and D in sequence according to a predetermined rule (see below). In the frame time Tf, the pixel 101 displays the average luminance of the sub-frames A′ and C, which may compare to or equal the luminance obtained in FIG. 1 when the gray-scale data A is directly displayed during the whole frame time Tf. Similarly, the pixel 102 displays the average luminance of the sub-frames B′ and D in the frame time Tf, which may compare to or equal the luminance obtained in FIG. 1 when the gray-scale data B is directly displayed during the whole frame time Tf.
As shown in FIG. 4, a look-up table 110 is an example of the aforementioned predetermined rule used in the method of FIG. 3. The table may be used to generate the sub-frame data. For example, two sub-frames with the gray-scale data of 250 and 0 in sequence will be output when a pixel receives the original gray-scale data of 150, and two sub-frames with the gray-scale data of 255 and 0 will be output in sequence when the pixel receives the original gray-scale data of 151. In the look-up table 110 of FIG. 4, when the original gray-scale value is smaller than 152, a second sub-frame with the gray-scale data of 0 (a black frame) and a corresponding first sub-frame are generated so that the resultant luminance effect of the two sub-frames is equal to the luminance of the original gray-scale value. When the original gray-scale value is greater than 151, a first sub-frame with the gray-scale data of 255 and a corresponding second sub-frame are added such that the resultant luminance effect of the two sub-frames is equal to the luminance of the original gray-scale value. Generally, the gray-scale values of the adjacent pixels 101 and 102 are very close to each other. Thus, if the original gray-scale values of the pixels 101 and 102 in FIG. 3 are smaller than 152, the gray-scale values C and D of the sub-frame are equal to 0. If the original gray-scale values of the pixels 101 and 102 are greater than 151, the gray-scale values A′ and B′ of the sub-frame are equal to 255. Each of these two conditions can effectively reduce the blur width of the dynamic image without influencing the luminance of displaying image.
In the second conventional technology when the image is continuously in the moving situation, replacing the black frame insertion with the gray frame insertion can indeed improve the flicker of the moving frame. However, when the image is changed from the moving situation to the motionless situation instantaneously, all the pixels originally driven by the dynamic mode are driven by the static mode. Consequently, the luminance of the whole image suddenly increases because no gray frame is inserted after the image is converted into the static one, and the viewer may notice the luminance surge in the generated image. Thus, the image display quality of the LCD device may be lessened.